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2 Once you make doing science with your VO service easy, everyone will want to use your server. Analagous to oversubscribed observatory time - how do users successfully compete for query time Query modelling in a proposal? Need data simulators/previewers to run query on. and/or data subset for test run.

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7 Regions with different sensitivity included in same source catalogues. c.f. XMM-Serendipitous Source Cat (created from pointed mode observations with different exposure times & instrument modes) Need a good coverage/sensitivity model of the data archive to understand volume of space contained in source catalogue. 6 6 binned image of RASS data set Survey depth

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10 Results in a sensitivity map of the RASS sky - adds usefulness to the source catalogue Doing this with RASS is straightforward (though not quick) as the total data archive is a few 10s of GB. Doing it for future observatories will have to be done on the archive curators server

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11 The role of Archive/Source Catalogue Metadata Data Archive Source Catalogue How should contents (not parameters) of a source catalogue best be described in the metadata? - why are the sources in it - in it? - describe the selection criteria X-ray photon lists/ancilliary instrument data Computationally expensive to reprocess Selection Criteria

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13 Other wavebands Similar challenges other wavebands. Complex coverage and sensitivity descriptions plus catalogue selection criteria. How many brown dwarves are there? In general, how much data description should go in the metadata and how much should be left in secondary resources?

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14 Final Questions. How big (Kbytes) should data archive metadata be? –Should it include preview data (e.g. large FITS files)? –Should selection criteria be described in the metadata (or simply a reference to the original publication) –Provide partially reduced or preview data as externally held addendum to the metadata? Much bigger than standard metadata Much smaller than whole archive –What other tools are needed to allow astronomers to assess usefullness of, justify to Time Allocation Committees large proposals/queries in a VO context?